Fix issues with std::filesystem::path settings

src/libutil/configuration.cc

@@ -433,10 +433,18 @@ std::string BaseSetting<StringMap>::to_string() const
         [](const auto & kvpair) { return kvpair.first + "=" + kvpair.second; });
 }
 
+static Path parsePath(const AbstractSetting & s, const std::string & str)
+{
+    if (str == "")
+        throw UsageError("setting '%s' is a path and paths cannot be empty", s.name);
+    else
+        return canonPath(str);
+}
+
 template<>
 std::filesystem::path BaseSetting<std::filesystem::path>::parse(const std::string & str) const
 {
-    return std::filesystem::path(str).lexically_normal();
+    return parsePath(*this, str);
 }
 
 template<>
@@ -445,6 +453,22 @@ std::string BaseSetting<std::filesystem::path>::to_string() const
     return value.string();
 }
 
+template<>
+std::optional<std::filesystem::path>
+BaseSetting<std::optional<std::filesystem::path>>::parse(const std::string & str) const
+{
+    if (str == "")
+        return std::nullopt;
+    else
+        return parsePath(*this, str);
+}
+
+template<>
+std::string BaseSetting<std::optional<std::filesystem::path>>::to_string() const
+{
+    return value ? value->string() : "";
+}
+
 template class BaseSetting<int>;
 template class BaseSetting<unsigned int>;
 template class BaseSetting<long>;
@@ -458,14 +482,7 @@ template class BaseSetting<StringSet>;
 template class BaseSetting<StringMap>;
 template class BaseSetting<std::set<ExperimentalFeature>>;
 template class BaseSetting<std::filesystem::path>;
-
+template class BaseSetting<std::optional<std::filesystem::path>>;
-static Path parsePath(const AbstractSetting & s, const std::string & str)
-{
-    if (str == "")
-        throw UsageError("setting '%s' is a path and paths cannot be empty", s.name);
-    else
-        return canonPath(str);
-}
 
 PathSetting::PathSetting(
     Config * options,

src/libutil/include/nix/util/file-path.hh

@@ -5,6 +5,7 @@
 
 #include "nix/util/types.hh"
 #include "nix/util/os-string.hh"
+#include "nix/util/json-non-null.hh"
 
 namespace nix {
 
@@ -53,4 +54,8 @@ std::optional<std::filesystem::path> maybePath(PathView path);
 
 std::filesystem::path pathNG(PathView path);
 
+template<>
+struct json_avoids_null<std::filesystem::path> : std::true_type
+{};
+
 } // namespace nix

src/libutil/include/nix/util/logging.hh

@@ -54,7 +54,7 @@ struct LoggerSettings : Config
           expression evaluation errors.
         )"};
 
-    Setting<std::filesystem::path> jsonLogPath{
+    Setting<std::optional<std::filesystem::path>> jsonLogPath{
         this,
         {},
         "json-log-path",

src/libutil/logging.cc

@@ -367,10 +367,10 @@ std::unique_ptr<Logger> makeJSONLogger(const std::filesystem::path & path, bool
 
 void applyJSONLogger()
 {
-    if (!loggerSettings.jsonLogPath.get().empty()) {
+    if (auto & opt = loggerSettings.jsonLogPath.get()) {
         try {
             std::vector<std::unique_ptr<Logger>> loggers;
-            loggers.push_back(makeJSONLogger(loggerSettings.jsonLogPath.get(), false));
+            loggers.push_back(makeJSONLogger(*opt, false));
             try {
                 logger = makeTeeLogger(std::move(logger), std::move(loggers));
             } catch (...) {